Currently, three subtitle formats are supported. 

1) Texas Instruments CineCanvas subtitles, used in Interop DCPs
2) SMPTE 428-7 “SubtitleReel” subtitles, used in SMPTE DCPs
3) W3C TTML (IMSC profiles), used in IMF packages

Limitations:  Only the most common attributes of the listed subtitle formats are evaluated for rendering.